The present application relates to liquid crystal display (LCD) technology.
The points discussed below may reflect the hindsight gained from the disclosed innovations, and are not necessarily admitted to be prior art.
Many applications require optically fast switching liquid crystal displays. One significant application is the elimination of motion blur in LCD television. The other is the use of field sequential color (FSC) to achieve full color display without the use of color filters.
There are several fast time switching LCD configurations, and one is the bend cell or pi-cell. It fulfills the most essential requirement of short switching time and good viewing angle (see P. J. Bos and K. R. Koehler/Beran, Mol. Cryst. Liq. Cryst., 113 (1984), p. 329). However, this bend mode is not stable under zero voltage bias, because the elastic energy of splay is always less than the bend mode under the same boundary conditions.
M. Xu et al., taught a method to stabilize the bend mode under zero bias using a very high pretilt angle (see M. Xu, D. K. Yang, P. J. Bos, SID Digest, 10, 2901 (1998)). A method to obtain different pretilt angle (0° to 90°) using a nano-texture alignment surface was introduced by Fion F. S. Yeung et al (see F. S. Y. Yeung, Y. W. Li and H. S. Kwok, Appl. Phys. Lett., 88, 041108 (2006)). In this method, polyamides for vertical and horizontal alignment are physically mixed to form sufficiently small domains on the alignment surface due to liquid-liquid phase separations. The pretilt angles are changeable according to different surface area ratio (See F. S. Y. Yeung, J. Y. Ho, Y. W. Li, F. C. Xie, O. Tsui, P. Sheng and H. S. Kwok, Appl. Phys. Lett., 88, 051910 (2006)). With high pretilt angles, the bend mode can be stable at zero voltage bias. This kind of stabilized bend mode is named “No-Bias Bend” mode. However, further efforts must be done in order to ensure the uniformity and robustness of the alignment surface.
Another method for fast switching LCD is the vertically aligned nematic LCD. The vertical alignment can provide excellent contrast (>1000:1). A high contrast is an important factor for performance of field sequential LCD. A high contrast ratio induces good color saturation and purity for color mixing. Otherwise, color leakage will affect the color reproduction. By using low velocity rotational viscosity liquid crystal (LC), and decreasing the cell-gap to about <2 μm, the switching time of the device can be as fast as 2 ms. However, such small cell-gaps are not favorable, and not feasible from manufacturing standpoint.
Prior art embodiments require the LCD to switch from one state to another within a very short time. For example, suppose the LCD alignment is in a certain steady state configuration at voltage V1 and another steady state configuration at voltage V2. It is then required that LC molecules change their alignment from one configuration to another quickly when the voltage is changed from V1 to V2. However, this is a stringent requirement that is not necessary if the backlight is a pulsed light such as a light emitting diode (LED). Presently, in most LCD applications, the frame rates are very fast. For example, in a 120 frame per second display, the frame time Tf is only 8 ms. In the case of field sequential color displays, the frame times are even shorter.
The present inventors have realized that it is an overkill to require the LC molecules to change their alignment within such a short time and stay in that configuration. To overcome the problems posed in the prior art, the present innovations use the transient response in conjunction with the pulsed backlight.
Previous approaches have been based on equilibrium effects. The LC is required to be stable under certain applied voltages. The optical response time is determined by the transit time between two different static steady states.
One particular transient effect is the optical bounce (see S H Chen et al., Flow effect in the chiral nematic liquid crystal cell, Journal of Applied Physics, vol 75, p 3491, 1999). The optical bounce is usually undesirable in LCD applications, but for this innovation, it is emphasized and enhanced. Note that optical bounce is only one of many possible transient effects. All of these effects are useful for the present innovations for producing a fast LCD.
In the present innovations, the transient effect of LC is used to produce a fast LCD response. Since the subframe time of the field sequential display is typically very short, the transient response is as good as the steady state response.
The grayscale is obtained by averaging the transmittance of the subframe. Using this dynamic approach, the true steady state-to-steady state response time of the LC can be ignored. The transient response time, in conjunction with the pulsed backlight is used, and it is found that the transient state can provide good transmittance and sufficient brightness for the display.
In these innovations, a special configuration of LC is used to maximize the brightness of the transient effect. Particularly, the cell-gap up to 5 μm can be used. Additionally, such configuration can be applied to drive the field sequential display using the passive matrix and the active matrix modes.